This invention relates to the production of metals. More particularly, it relates to the detection of the presence of molten salts in molten metals, such as Al or Al alloys.
In order to produce high quality metals, in particularly Al and Al alloys, filtration of the molten metals is employed to remove particulate contaminants or impurities. Those contaminants or impurities are typically removed by the use of ceramic filters. One such filter is described in U.S. Pat. No. 4,056,586 assigned to SELEE Corporation, assignee of this application.
In addition, other impurities, such as Na, Ca and H.sub.2, are removed from the molten metal by adding reaction gases to react with those impurities which are more electrically positive than the Al. Often Ar, Cl.sub.2, or F.sub.2 gases and/or mixture thereof are injected into the molten Al alloys before filtration and casting in order to remove H.sub.2 and to facilitate the floatation of solid impurities to the surface and their incorporation into the dross.
It is often difficult, however, for the operator of the metal production process to closely regulate the gas injection, and as a result, excess Cl.sub.2 or F.sub.2 can react with metals, such as Al and Mg to form salts. The existence of salts is particularly a problem where the salts are in a molten state at the temperatures of molten Al. NaCl and CaCl.sub.2 are solids at molten Al temperatures, i.e. 750.degree. C. and may be filtered from the molten Al by the ceramic filter. However, MgCl.sub.2 and certain eutectic combinations of chlorides are in a liquid or molten state at molten Al temperatures and will not be filtered from the molten Al through conventional filters. These molten salts, which are suspended in the Al, become contaminant liquid or molten salts in the molten metal. The molten salts not only adversely affect the properties of the final Al alloy, but also reduce the efficiency of the filtration since the molten salt droplets adhere to solid impurities and prevent their collection by the filter and promote their release from the filter. This process of forming contaminant salts is sometimes referred to as chloridizing.
Applicant is presently unaware of the existence of a simple, inexpensive and reliable apparatus or method which can alert the operator when molten salt concentrations exceed maximum allowable limits established by metal producers.
One time consuming and labor intensive method of detecting these molten salts is done by withdrawing a sample of the molten metal, allowing it to cool and solidify, and cutting out a section. The section is analyzed by using a standard chemical or physical analytical technique. Obviously, such procedure is too time consuming. Typically, production cannot be stopped or delayed until an analysis is obtained.
U.S. Pat. No. 5,226,950 issued to Yu recognizes the problems associated with molten metal salts in molten Al. The Yu patent provides a method for detecting the presence of molten salts in the molten metal by a complex process. Yu calls for the introduction of a substantially inert gas into the molten metal, measuring the bubble size of the bubble formation of the gas in the metal, introducing a second liquid element into the metal to form a combination liquid, introducing a substantially inert gas into the combination liquid, measuring the bubble size formation of the gas in the combination liquid, and comparing the gas bubble size of the bubble formation of the first liquid with the gas bubble size of the bubble formation in the combination liquid. The Yu patent requires complex measuring apparatus to measure bubble frequency and bubble size.